terized by rapid cooling in fall, with subsidence of many kilometers associated with the establishment of a strong polar jet that restricts exchange. This jet confines the winter polar stratosphere, most acutely in the southern hemisphere, and plays a significant role in the annual dynamical cycle of the stratosphere. The subtropical jet, the respective polar jets, and the tropopause constitute barriers to exchange; the low and high latitudes in each hemisphere are to a degree dynamically coupled in the lower stratosphere. Predicting accurately the path taken by material from a given point in the stratosphere in a given season is a central unanswered question. Our understanding of the response of the stratosphere to natural and inflicted changes is seriously compromised by this lack of understanding.
In formulating a strategy for studying the stratosphere, we have identified five basic scientific questions that we believe will motivate research on stratospheric ozone in the coming decades (see Box 5.1). The essential research activities that will be required to address these questions are outlined later in the section on research imperatives.
Observed increases in the concentrations of CO2, CH4, N2O, and CFCs provide one of the clearest manifestations of global change in the atmosphere. Historical trends in H2O and O3 have yet to be quantitatively characterized. However, limited data suggest that tropospheric ozone concentrations may have
BOX 5.1 Stratospheric Ozone: Key Scientific Questions